Display a grapic - I know it's simple, but I just can't get it to work

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Display a grapic - I know it's simple, but I just can't get it to work

Right now I've got a terrible sinus condition so my brain probably isn't functioning properly. I'm hoping someone will take pity on me and help me out.

I've got a simple HTML page used to specify files for uploading. The uploading is done with a PHP routine. It works fine except that larger files take a long time. I just want to display an animated 'wait' gif while the php file processes. I want it to display in the middle of the page (not in a new window) when the Submit button is clicked (after checking to make sure the userid and password have been entered). I know that's simple, but I just can't get it to work. Here's the HTML code. I've put in a comment where (when) I want the graphic to display.